Extrusion dies are frequently used to provide a tubular coating to a wire or other product which provides a generally cylindrical substrate. A typical die assembly 101 for performing this function is shown in FIG. 1. The basic assembly 101 consists of a die body 102, a tip 103, a die holder 104, and a die 105 held in place by end caps 106 and 114. The tip 103 is mounted within an axial bore of the die body 101 and is itself constructed with an axial bore 107 having a downstream exit portion 108. In operation, a wire or other tubular product 116 is fed through bores 107 and 108 to exit axially at 115. An annular extrusion passage 109, surrounds exit 115, and is positioned to receive flowing plastic and apply it, in a tubular layer, to the product 116 at exit 115.
The die assembly 101 is an assembly of machined parts, each having its own manufacturing tolerance. These tolerances tend to multiply with the assembly of the components. It is therefore a difficult task to maintain the desired coaxial relationship between the product and the extruded layer. An adjustment mechanism is generally needed to insure concentricity of the die and pin so as to provide an even thickness of the applied layer. This is generally accomplished simply by mounting the die holder 104 for adjustment, along x and y coordinates. The adjustment may be actuated through adjusting screws 112. As shown in FIG. 1, the adjustment motion of the prior art is radially towards and away from the axis 117, of the assembly 101 as a torque is applied to an adjustment screw 112. This may require a significant adjustment torque because of the high thrust loads on the die holder during operation.
The die assembly of this invention may be generally described as a cross head die in that it receives molten plastic from an extruder which enters the die passages in a direction that is transverse to the longitudinal axis of the assembly. The plastic must then be turned to flow downstream axially towards an annular tapered extrusion channel. The extrusion channel surrounds an axially extending passage through which a wire or tubular product may be directed to receive a cylindrical layer from the extrusion passage. In order to apply a cylindrical layer of constant radial depth, the position of the die relative to the tip must be precise. Since the degree of positioning accuracy cannot be maintained during assembly of the components, an adjustment mechanism is provided which allows the relative position of die and tip to be adjusted after assembly.
The die body of this invention is constructed with a recess at its downstream end to receive the die holder which supports the die in a fixed relation. The die holder and die body recess engage to allow a pivot motion between the two components. To accomplish the pivot motion, the recess is constructed with a spherical internal surface which mates with a spherical external surface constructed on the die holder. These surfaces engage and operationally cooperate to allow the die holder to pivot in a substantially universal motion. To actuate the adjustment, adjusting screws are provided in the die body which engage the die holder at a position axially displaced from the spherically engaged surfaces of the die body and die holder. This provides a mechanical advantage proportional to the distance that the adjustment screws are displaced from the fulcrum formed by the spherical surfaces and results in a reduction of the torque which must be exerted on the adjustment screws to correct the relative position of the die and tip. In this manner the die and tip may be accurately maintained in a concentric relation.